1. Field of the Invention
This invention relates to on-chip multispectral imaging and more specifically to the incorporation of photonic crystals and integrated processing capability with a focal plane array to provide spectral tunability and spatial agility in the form of an Adaptive Focal Plane Array (AFPA).
2. Description of the Related Art
Multispectral imagers flown on aircraft and satellite platforms have provided a wealth of information, ranging from military strategic targets to crop assessments. Multispectral image processing has demonstrated high target detection with low false-alarm rates. However, multispectral imagers suffer from limitations, including high mass, volume, and power requirements, as well as a “fire hose” of data to process off line. Transitioning multispectral imaging technology to a tactical sensor or small commercial package in which data is processed in real time will require intelligent on-chip data management. If the imager were capable of adapting spectrally and spatially on a pixel-by-pixel basis, only those wavelengths and segments of the image that are important in the scene would need to be further processed and transmitted to the user.
Historically, multispectral sensors are full-frame. None of these approaches are suitable for the necessary miniaturization needed for pixel-by-pixel tuning. Full-frame approaches include, for example, placing spectral filters in a spinning disc in front of a broadband Focal Plane Array (FPA), or using multiple detectors, each detector having its own spectral filter. The first scheme yields images in each passband at a slow rate, usually exhibiting target blurring due to motion. Also this method results in poor wavelength selectivity and provides, at most, only a few different wavebands. In the latter approach, drawbacks include detector-to-detector image registration requirements and complex opto-mechanical assemblies, and high data rates. Also, tunable liquid-crystal (LC) filters are available commercially. At least one version implements an LC device within a Fabry-Perot cavity to provide tuning over a range of wavelengths (0.4–1.7 microns) suitable for use in the telecommunications industry. Another version stacks LC-based retarders in series to provide wavelength tunability over 0.4–0.7 microns. LCs are generally useful only at wavelengths below 2 microns. The semiconductor band structure of the detector material can be engineered to achieve voltage-controlled tunability. However, this approach yields only a narrow range of tuning, and wavelength selectivity can be poor due to spectral bleeding. These systems are generally wideband, not narrowband. An approach which cascades detectors of varying responses suffers from successive shielding as the number of layers increases, limiting the number of layers to two or three.
At the present time, we are aware of two proposed approaches for pixel-by-pixel tunability. Neither approach will achieve full-spectrum tuning or high-resolution wavelength selectivity. In the first approach, a tunable Fabry-Perot (FP) interference filter is placed in front of each pixel of the FPA, with tuning achieved by translation of one of the plates. Implementation of a tunable FP filter must address issues such as the area required for the actuators, suppression of unwanted orders, and the placement of the FP filter in collimated space due to a paraxial limitation. Researchers have built FP resonators with a typical tuning range of only approximately 7% (J. Peerlings et al., “Long Resonator Micromachined Tunable GaAs—AlAs Fabry-Pérot Filter,” IEEE Photonics Technology Letters 9, 1235 (1997)).
In the second approach, tunable Fresnel Zone Plates (FZPs) are used to create a multispectral AFPA. The FZP acts as a highly dispersive lens, and can be used in conjunction with a confocal pinhole to provide narrowband wavelength selection. By tuning the focal length of the zone plate (by lateral actuation of the zone plates using microelectromechanical technology), one can “scan” over a wavelength range of interest. There has been successful fabrication of FZPs using structures similar to tunable gratings. However, the scanning range is limited by the actuation technology—the devices are typically tunable by only 1–10% around the nominal unactuated value.